The present invention relates to a method of forming a plastic molded article, and more particularly to a method of forming a plastic molded article which has surfaces each of which is essentially asymmetrical between the right and left halves.
An example of a plastic molded article which has surfaces which are asymmetrical between the right and left halves is a synthetic resin shutter which is slidably engaged with the casing of a magnetic disc cartridge in such a manner as to open and close the opening which is formed in the casing so as to receive the magnetic head.
A magnetic disc cartridge, as shown in FIG. 1, includes a casing 52 in which a magnetic disc 53 is rotatably accommodated, and a shutter 56 for closing and opening an opening 54 formed in the casing 52 so as to receive the magnetic head. The shutter 56, as shown in FIGS. 1 and 2, is composed of a pair of main plates 57 which are parallel with each other and have a shutter window 60, respectively, and a coupling plate 59 through which the main plates 57 are coupled to each other. The coupling plate 59 slides along the front edge of the casing 52. That is, the shutter 56 is slidably engaged with shallow recesses 58 formed in the casing 52 near the magnetic head insertion opening 54, so that it can be moved between an opening position where the opening 54 is opened and a closing position where the opening 54 is closed.
In general, the shutter 56 is U-shaped in section, as shown in FIG. 2, and is made of metal such as stainless steel or aluminum. The shutter windows 60 which are substantially equal in size to the opening 54 of the casing 52. The shutter 56 is urged by a spring (not shown) in such a manner that normally it is set at the closed position, thereby to protect the magnetic disc and to prevent the entrance of dust or the like into the casing 52.
When the magnetic disc cartridge is in use, the shutter 56 is moved to the open position, that is, the shutter windows 60 are moved to the opening 54 to allow the insertion of the magnetic head into the opening 54.
Recently, a magnetic disc cartridge has been proposed which employs a shutter which is made of synthetic resin instead of a metal such as stainless steel.
In forming the synthetic resin shutter 56 by injection molding, the position of the gates for injection is generally at one end or both ends of the coupling plate 59, or at the middle of the latter. Since the shutter 56 is small in wall thickness, high pressure is required to form the shutter 56 by injection molding. Moreover, with the above-described conventional injection positions, the molding material is not uniformly spread in the mold cavity; that is, first the central portion, becoming high in pressure, is filled with the material, and thereafter the remaining portions around the central portion are filled with the material. Thus, the injected material is liable to be non-uniform in thickness and include shrinkage cavities.
Although a difficulty encountered in the formation of the shutter of the magnetic disc cartridge has been described, it should be noted that the same difficulty is generally involved in the formation of plastic products which have surfaces which are asymmetric between right and left halves.
The present invention further relates generally to a disc cartridge. More particularly, the present invention relates to a disc cartridge including a shutter slidably mounted on a cartridge case in which a disc-shaped recording medium is received, and to a method for producing such a disc cartridge.
In view of convenience during practical use and reliable protection from dust or similar foreign material, a disc-shaped recording medium such as a magnetic disc, optical disc, photomagnetic disc or the like is constructed such that it is loaded in a recording/reproducing apparatus as it is received in a comparatively rigid cartridge case so as to allow desired information to be recorded or reproduced.
A 3.5 inch micro floppy disc cartridge as shown in FIG. 3 has been hitherto known as a cartridge of the foregoing type. Specifically, a cartridge 21 is constructed such that a magnetic disc 5 is rotatably received in rectangular upper and lower half shells 2 and 3 molded of an acrylonitrile-butadiene-styrene resin or the like. The magnetic disc 5 is a flexible magnetic recording medium which has a magnetic layer uniformly formed on the surface of a circular disc-shaped film base molded of a high molecular organic material. In addition, a ring-shaped center core 4 is fixedly fitted to a circular opening formed at the central part of the magnetic disc 5. An opening 7 is formed at the central part of the lower half shell 3 of the cartridge 21 so as to allow a motor shaft to be introduced into a substantially square motor shaft insertion hole 6 formed at the central part of the center core 4. Additionally, magnetic head insert opening portions 8 each opened forwardly of the cartridge 21 are formed on the upper and lower half shells 2 and 3 so as to allow a magnetic head and a head pad to be introduced into the magnetic head insert opening portions 8, respectively, for writing or reading a number of predetermined information using the magnetic disc 5.
To reliably prevent introduction of dust or the like into the interior of the cartridge 21 via the magnetic head insert opening portions 8 as well as adhesive deposition of the dust or the like on the magnetic disc 5, the cartridge 21 is provided with a slidable type shutter 22 for the purpose of opening and closing the magnetic head insert opening portions 8.
The shutter 22 has been hitherto formed by bending a metallic material sheet such as a stainless steel plate or the like in an U-shaped cross-sectional contour with the aid of a press or the like. The shutter 22 includes projections at the base end region thereof in such a manner that slidable displacement of the projections in the opening/closing direction of the magnetic head insert opening portions 8 is properly guided along guide grooves formed along the front edge of the lower half shell 3 on the outer surface of the same. Specifically, the shutter 22 slidably moves within a slidable displacement region 11 extending on one side surface of the cartridge 21 inclusive of the peripheral edge of each of the magnetic head insert opening portions 8. In addition, the shutter 22 is formed with an opening portion 23 which coincides in position with the magnetic head insert opening portions 8 so as to allow the magnetic disc 5 to be exposed to the outside when the cartridge 21 is in use. When the cartridge 21 is not in use, the magnetic head insert opening portions 8 are closed with the shutter 22 and the magnetic disc 5 is covered with the shutter 22, as represented by phantom lines in FIG. 3.
As stated above, in recent years, proposals have been made as to a shutter molded of a synthetic resin in an integral structure instead of the conventional shutter made from a metallic material plate as a stainless steel plate or the like, as disclosed in, e.g., Japanese Unexamined Patent Publication (Kokai) No. 60-231985 and Japanese Unexamined Patent Publication (Kokai) No. 64-70981. Each of these prior structures provides a shutter construction which assures that the shutter can easily be molded of an inexpensive resin instead of the conventional shutter made of a metallic material plate and which requires many production steps accompanied by significant production difficulties.
In a case where the shutter is molded from a synthetic resin in an integral structure, the resin flows against a large resistance due to the thin wall of the shutter during the injection molding operation, and the moldability of the molten resin is undesirably degraded because of the flow resistance itself. The problem of degraded injection moldability appears largely dependent on the position where a gate is formed for the purpose of injecting the molten resin into the hollow molding cavity used for the injection molding operation.
To obviate the foregoing problem, a proposal has been made as to a shutter wherein a gate is formed on a coupling plate 37 by way of which a main plate 35 is connected to an other main plate (located opposite to the lower plate 35). This proposal results in a raised gate mark G.sub.1 appearing on the outer surface of the coupling plate 37, as shown in FIG. 3. According to this proposal, the resultant shutter has advantages that the gates can easily be formed on a molding die, the molten resin injected toward the hollow space in the mold cavity can substantially simultaneously reach the end region of the hollow space in the mold cavity, and flaws recognizable on the shutter such as stripes indicative of irregular molding or the like can be reduced.
However, it has been found that a shutter molded of a synthetic resin has a problem in that the raised gate mark G.sub.1 remaining after completion of the molding operation for the shutter is located on the outer surface of the coupling plate 37, which is often incompletely treated. In addition, the foregoing problem is sometimes associated with engagement of the raised gate marks G.sub.1 with a position-determining projection 41 for the cartridge 21 at the time when the cartridge 21 is loaded in a recording/reproducing apparatus. When the cartridge 21 is loaded in the recording/reproducing apparatus, it is inserted into a slot in the recording/-reproducing apparatus in the direction of the arrow D in FIG. 3. At this time, the shutter 22 is slidably displaced in such a direction that the magnetic head insert openings 8 is made to coincide in position with the opening portion 23 (in the direction of the arrow E) with the aid of an actuating member arranged in the recording/reproducing apparatus. To restrict the position where the cartridge 21 can be inserted into the slot, the coupling plate 37 is slidably displaced relative to the position-determining projection 41 disposed on the deeper side of the insertion slot of the recording/reproducing apparatus.
As the coupling plate 37 is slidably displaced in the above described manner, the raised gate mark G.sub.1 is displaced such that the raised gate mark G.sub.1 located leftward of the position-determining projection 41 as seen in FIG. 3 moves to a position G.sub.2 located rightward of the position-determining projection 41. However, if the raised gate mark G.sub.1 undesirably engages with the position-determining projection 41 in the course of the slidable displacement of the coupling plate 37, part of the raised gate mark G.sub.1 is scraped, thereby producing pulverized synthetic resin. Sometimes, the opening operation of the shutter being is performed incorrectly because of the pulverized synthetic resin.